1. Field of the Invention
The present invention relates to a linear actuator and, in particular, to a novel linear-actuator construction in which a screw shaft portion is integrally formed at the distal end of the rotation shaft of the motor to eliminate the coupling conventionally required and in which one of the conventional pair of support bases is omitted, thereby substantially simplifying the structure and achieving an improvement in terms of positioning in linear motion and efficiency in power transmission.
2. Description of the Related Art
FIG. 3 shows the construction of a conventional linear actuator of this type generally adopted.
In FIG. 3, reference numeral 1 indicates a motor composed of a step motor or the like. The motor 1 has a rotation shaft 2 connected to a screw shaft portion 4 through the intermediation of a coupling 3 composed of a joint.
The screw shaft portion 4 is supported at its ends 4a and 4b by support bearings 5a and 6a of a pair of support bases 5 and 6. Threadedly engaged with this screw shaft portion 4 is a nut portion 10 constituting a part of the associated stage or table.
In the above-described construction, rotating the screw portion 4 through operation of the motor enables the nut portion 10 to make a reciprocating linear movement in the direction indicated by the arrow A.
The conventional linear actuator, constructed as described above, has the following problems.
In this construction, the rotation shaft of the motor is connected to the screw portion formed separately from the rotation shaft through the intermediation of the coupling, so that the coupling is indispensable, which leads to a rather large number of parts, resulting in high cost. Further, the intermediation of the coupling leads to a rather low efficiency in positioning in linear motion.
Further, since the screw shaft portion is formed as a component separate from the motor rotation shaft, it has to be rotatably supported by a pair of support bases, which requires a rather large space and makes it difficult to achieve a reduction in the number of parts.
The present invention has been made with a view toward solving the above problems in the prior art. It is accordingly an object of the present invention to provide, in particular, a linear actuator in which the screw shaft portion is integrally formed at the distal end of the rotation shaft of the motor to eliminate the coupling conventionally required and in which one of the conventional pair of support bases is omitted, thereby substantially simplifying the structure and achieving an improvement in terms of positioning in linear motion.
According to the present invention, there is provided a linear actuator including: a screw shaft portion formed on a distal end side of a rotation shaft of a motor; and a nut portion threadedly engaged with the screw shaft portion, in which the nut portion is capable of making a linear reciprocating movement through rotation of the screw shaft portion, and in which the screw shaft portion and the rotation shaft are formed as an integral unit using the same material in a seamless state. In the linear actuator described above, the screw shaft portion has a second diameter which is larger than a first diameter of the rotation shaft. Further, in the linear actuator, the motor is equipped with a first bearing portion composed of a single bearing and a second bearing portion composed of two bearings, and the rotation shaft has a small diameter portion borne by the first bearing portion and a large diameter portion borne by the second bearing portion and having a diameter larger than that of the small diameter portion. Further, in the linear actuator the large diameter portion has a diameter which is the same as the second diameter of the screw shaft portion. Further, the linear actuator has a structure in which the motor is composed of a step motor. Furthermore, in the linear actuator, a step portion is formed by the large diameter portion at a boundary between the small diameter portion and the large diameter portion, and a front end of a rotor provided on the rotation shaft abuts the step portion to undergo positioning. In addition, the linear actuator has a structure in which the rotor is composed of a plurality of magnet plates and a plurality of yoke plates stacked together, and the rotor is situated exclusively on the small diameter portion. Moreover, in the linear actuator, a distal end portion of the screw shaft portion is supported exclusively by a single support base.